The present invention concerns the field of solid state capacitors and relates particularly to massed production methods for manufacturing solid state capacitors.
A massed production method for solid state tantalum capacitors is described in U.S. Pat. No. 5,357,399 (inventor Ian Salisbury). This method involves providing a substrate wafer of solid tantalum, forming a sintered, highly porous, layer of tantalum on the substrate, sawing the layer of porous tantalum with an orthogonal pattern of channels to produce an array of upstanding porous tantalum rectilinear bodies, anodising the bodies to produce a dielectric layer thereon, dipping the bodies in manganese nitrate solution and heating to convert the applied solution to manganese dioxide thereby to form a cathode layer, cutting along the channels to isolate the cathode and anode ends of individual upstanding bodies by removing the manganese dioxide connecting adjacent bodies, reforming a dielectric layer on the newly exposed tantalum substrate revealed by the isolation cut, applying respective conducting layers of carbon and then silver onto top ends of each body, bonding a lid consisting of a wafer of solid metal onto the silver layer, injecting insulating resin material into the channels between bodies constrained by the substrate and lid, and slicing the assembly in a direction perpendicular to the lane of the wafers and along the centre line of each channel thereby to produce a plurality of capacitors in which the anode terminal consists of substrate material, the cathode terminal consists of lid material and the capacitive body consists of the coated porous tantalum body.
This method allows the production of highly compact, reliable capacitors of a high volumetric efficiency. However with the continued miniaturization of components demanded by the electronics industry there is pressure to produce ever smaller and more efficient capacitors.
Our recent PCT publication WO 00/285 describes a modified version of the Salisbury process. In this version the volumetric efficiency of the individual capacitor bodies is increased by forming cathode terminals directly on the silver/carbon coated top ends of the porous capacitor bodies. In this way the solid lid layer applied in the Salisbury method may be omitted. The volume which would have been occupied by the solid lid layer is thus available to increase the size of the porous body. Hence for a given capacitor body size, a greater volume of capacitive material can be included.
There is continuous pressure for efficiency improvements in these manufacturing processes. The present invention seeks to provide a simplification of multiple capacitor manufacturing processes of the type described in the foregoing, thereby to provide an economic advantage which results in cheaper capacitors.
According one aspect of the present invention there is provided a method of manufacturing multiple solid state capacitors comprising:
providing a substrate layer;
forming on an upper surface of the substrate layer a plurality of upstanding bodies consisting of porous sintered valve-action material;
forming a dielectric layer on the bodies;
applying an electrically insulating resist layer to the dielectric layer present in a region separating adjacent upstanding bodies,
forming a cathode layer on the exposed dielectric layer on the upstanding bodies and on the resist layer between the bodies;
applying a cathode terminal to an upper end region of each upstanding body;
dividing the processed substrate into a plurality of individual capacitor bodies each having an anode terminal portion at one end comprising divided substrate, a capacitive portion comprising one of the porous bodies and a cathode terminal portion at the other end.
The application of an insulating resist layer after formation of the dielectric layer, and before formation of the cathode layer, considerably simplifies the isolation of anode and cathode portions of each capacitor body. The prior art isolation method involves grinding through both cathode and dielectric layers in the regions between adjacent bodies, thereby exposing fresh substrate. The dielectric layer on the freshly exposed substrate must then be re-formed by a second dielectric formation process. The present invention provides a process in which a single dielectric layer formation process is carried out. Because the, resist layer masks the dielectric layer in between adjacent bodies, no grinding through to the substrate is necessary. Hence the isolation step is considerably simplified, reducing production time and cost.
The method may further include a step in which art least a portion of the resist layer coated in cathode layer is removed. The removal may be by, mechanical, chemical or another suitable means. Preferably a portion of the resist layer around each upstanding body is removed, thereby to form a border surface free of cathode layer material around each body. En one aspect of the invention a shallow skim of the resist layer is made by machining. The skimming may be conducted by means of a cutting wheel or saw. The cut should not extend into the resist layer as far as the dielectric layer underneath. In this way the integrity of the dielectric layer is maintained.
The resist material must adhere well to the dielectric layer and form an intimate contact therewith in order to prevent contamination of the resist coated dielectric layer by cathode layer material. The resist layer may be applied as a permanent feature or a temporary feature. For example, the permanent resist layer will be retained in the structure of the capacitors which are produced by the method of the invention. Typical resist materials are fluoro-polymers and epoxy resins. An additional benefit of the permanent resist layer may be achieved by incorporating a colour or contrast in the resist which stands-out relative to the encapsulation material. In this way the final capacitors are provided with an orientation indication indicative of capacitor polarity, with the resist layer corresponding to the anode end.
A temporary resist layer would be applied prior to the manganising process (cathode layer formation) and removed before encapsulation. The temporary resist may be a photolithographic resist, a chemically detachable resist or a mechanically removable resist.
In the encapsulation process, an insulating material may be infilled between the cathode-coated and terminated capacitor bodies. When the substrate is subsequently divided the infilled insulating material forms a protective sleeve around a mid portion of the capacitor, leaving only the anode and cathode terminal features exposed. The encapsulation material is preferably a plastics resin material, in particular a setting epoxy resin.
The encapsulation may involve a preliminary stage in which e.g. powdered thermoplastics resin is introduced into the spaces between the upstanding bodies and then melted by heating of the substrate to form a layer of thermoplastic partway up the sides of each body. Preferably this preliminary part-encapsulation is conducted using a resin of different coloration to the main encapsulation resin, thereby providing a visible polarity indication in the final capacitors. Alternatively polarity may be indicated by other marking such as laser etching.
The capacitor bodies may be formed into useful capacitors by a termination process in which the respective exposed cathode and anode surfaces of each capacitor body are liquid or vapor phase coated with a termination material which facilitates electrical connection of the respective ends of the capacitor to an electric circuit.
The respective terminal coatings may form a cap on each end of the capacitor body, as in the industry-standard five-sided termination processes.
In certain preferred embodiments the valve-action material is tantalum. However other valve-action materials may be used in the process of the present invention. These may be metals or non-metals, the essential characteristic being a capacitor forming capability. Examples of suitable metals niobium, molybdenum, aluminium, titanium, tungsten, zirconium and alloys thereof. Preferred examples are niobium and tantalum.
When the valve action metal is tantalum the substrate is preferably a solid tantalum wafer, thereby ensuring physical and chemical compatibility with the porous metal.
The upstanding anode bodies may be formed by a process which involves pressing a layer of valve-action material powder onto the substrate and sintering to fuse the powdered particles. Typically a seeding layer of coarse grade powder may have to be applied to the substrate and sintered thereto before finer grade powder is pressed onto the substrate. The coarse grade powder provides mechanical keying ensuring that a strong connection between the substantive porous layer and the substrate is produced. The strong connection is necessary to ensure that separation of the porous layer from the substrate does not occur during subsequent steps in the manufacturing process. The coherent layer of porous valve action metal thereby produced may be machined or otherwise processed to produce the individual anode bodies. The bodies may be formed by machining of a porous sintered layer formed on the substrate. The machining may be by means of orthogonal sawing to form rectilinear bodies. Other body formation process may be used, for example a combined pressing and casting of individual bodies, as described in our co-pending application number 9918852.6.
The dielectric layer may be formed by an electrolytic anodization process in which an oxide film is gradually built up on the surface of the porous sintered anode body. Suitable methods will be known to the person skilled in the art.
The cathode layer may be formed by dipping the anode bodies into a cathode layer precursor solution such as manganese nitrate and then heating to produce a cathode layer of manganese dioxide. Repeated dipping and heating steps may be carried out in order gradually to build up the required depth and integrity of cathode layer.
With the application of the cathode layer, the anode body becomes a capacitive body comprising an anode portion consisting of an interconnected matrix of: valve acting powder; dielectric insulating layer of valve acting oxide; and a conducting cathode layer of doped oxide.
According to a further aspect of the invention there is provided a capacitor produced by any method hereinbefore described.
According to another aspect of the invention there is provided an electronic or electrical device comprising a capacitor made by any method hereinbefore described.